Process and composition for rapid mass production of holographic recording article

ABSTRACT

A rapid mass production of the high performance holographic recording articles including the process and composition are described. To prepare a high performance holographic recording article based on two-component urethane matrix system, for example, polyols and all the additives must be virtually free of moisture contents. Deaeration must be carried out, once isocyanate and polyols including catalysts and all other ingredients are mixed together, to eliminate all entrapped air that is introduced into the mixture during mixing. The deaeration takes time, and the urethane reaction must not be allowed to take place until all air bubbles are evacuated from the isocyanate-polyols mixture. The rapid mass production of this invention overcomes such process limitations and results in a high-volume production of the high performance holographic recording articles.

RELATED APPLICATION

[0001] This application claims priority from U.S. ProvisionalApplication No. 60/310,225 filed Aug. 7, 2001, which is entitled thesame as this application.

FIELD OF THE INVENTION

[0002] The invention relates to optical articles including holographicrecording media, in particular media useful either with holographicstorage systems or as components such as optical filters or beamsteerers. In particular, this invention relates to rapid mass productionof high performance holographic recording article.

BACKGROUND

[0003] Developers of information storage devices and methods continue toseek increased storage capacity. As part of this development, so-calledpage-wise memory systems, in particular holographic systems, have beensuggested as alternatives to conventional memory devices.

[0004] A hologram stores data in three dimensions and reads an entirepage of data at one time, i.e., page-wise, which is unlike an optical CDdisk that stores data in two dimensions and reads a track at a time.Page-wise systems involve the storage and readout of an entiretwo-dimensional representation, e.g., a page, of data. Typically,recording light passes through a two-dimensional array of dark andtransparent areas representing data, and the holographic system stores,in three dimensions, holographic representations of the pages aspatterns of varying refractive index imprinted into a storage medium.Holographic systems are discussed generally in D. Psaltis et al.,“Holographic Memories,” Scientific American, November 1995, thedisclosure of which is hereby incorporated by reference. One method ofholographic storage is phase correlation multiplex holography, which isdescribed in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, thedisclosure of which is hereby incorporated by reference.

[0005] The advantages of recording a hologram are high density (storageof hundreds of billions of bytes of data), high speed (transfer rate ofa billion or more bits per second) and ability to select a randomlychosen data element in 100 microseconds or less. These advantages arisefrom three-dimensional recording and from simultaneous readout of anentire page of data at one time.

[0006] A hologram is a pattern, also known as a grating, which is formedwhen two laser beams interfere with each other in a light-sensitivematerial (LSM) whose optical properties are altered by the intersectingbeams. Before the bits of data can be imprinted in this manner in theLSM, they must be represented as a pattern of clear and opaque squareson a display such as a liquid crystal display (LCD) screen, a miniatureversion of the ones in laptop computers. Other devices such asreflective LCD's or reflective deformable micromirror devices can alsobe used to represent the data. A blue-green laser beam, for example, isshined through this crossword-puzzlelike pattern called a page, andfocused by lenses to create a beam known as a signal beam. A hologram ofthe page of data is created when the signal beam meets another beam,called the reference beam, in the LSM. The reference beam could becollimated, which means that all its light waves are synchronized, withcrests and troughs passing through a plane in lockstep. Such waves areknown as plane waves. The reference beam may also be a spherical beam ormay be phase-encoded or structured in other manners well known in thefield of holography. The grating created when the signal and referencebeams meet is captured as a pattern of varying refractivity in the LSM.

[0007] After recording the grating, the page can be holographicallyreconstructed by for example shining the reference beam into the LSMfrom the same angle at which it had entered the LSM to create thehologram. As it passes through the grating in the LSM, the referencebeam is diffracted in such a way that it recreates the image of theoriginal page and the information contained on it. A reconstructed pageis then projected onto a detector such as an array of electroopticaldetectors that sense the light-and-dark pattern, thereby reading all thestored information on the page at once. The data can then beelectronically stored, accessed or manipulated by any conventionalcomputer.

[0008] In one embodiment of phase correlation multiplex holography, areference light beam is passed through a phase mask, and intersected inthe recording medium with a signal beam that has passed through an arrayrepresenting data, thereby forming a hologram in the medium. The spatialrelation of the phase mask and the reference beam is adjusted for eachsuccessive page of data, thereby modulating the phase of the referencebeam and allowing the data to be stored at overlapping areas in themedium. The data is later reconstructed by passing a reference beamthrough the original storage location with the same phase modulationused during data storage. It is also possible to use volume holograms aspassive optical components to control or modify light directed at themedium, e.g., filters or beam steerers. Writing processes that providerefractive index changes are also capable of forming articles such aswaveguides.

[0009] The capabilities of holographic storage systems are limited inpart by the storage media. Iron-doped lithium niobate has been used as astorage medium for research purposes for many years. However, lithiumniobate is expensive, exhibits poor sensitivity (1 J/cm²), has low indexcontrast (Δn of about 10⁻⁴), and exhibits destructive read-out (i.e.,images are destroyed upon reading). Alternatives have therefore beensought, particularly in the area of photosensitive polymer films. See,e.g., W. K. Smothers et al., “Photopolymers for Holography,” SPIEOE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The materialdescribed in this article contains a photoimageable system containing aliquid monomer material (the photoactive monomer) and a photoinitiator(which promotes the polymerization of the monomer upon exposure tolight), where the photoimageable system is in an organic polymer hostmatrix that is substantially inert to the exposure light. During writingof information into the material (by passing recording light through anarray representing data), the monomer polymerizes in the exposedregions. Due to the lowering of the monomer concentration caused by thepolymerization, monomer from the dark, unexposed regions of the materialdiffuses to the exposed regions. The polymerization and resultingconcentration gradient create a refractive index change, forming thehologram representing the data. Unfortunately, deposition onto asubstrate of the preformed matrix material containing the photoimageablesystem requires use of solvent, and the thickness of the material istherefore limited, e.g., to no more than about 150 μm, to allow enoughevaporation of the solvent to attain a stable material and reduce voidformation. In holographic processes such as described above, whichutilize three-dimensional space of a medium, the storage capacity isproportional to a medium's thickness. Thus, the need for solvent removalinhibits the storage capacity of a medium. (Holography of this type istypically referred to as volume holography because a Klein-Cook Qparameter greater than 1 is exhibited (see W. Klein and B. Cook,“Unified approach to ultrasonic light diffraction,” IEEE Transaction onSonics and Ultrasonics, SU-14, 1967, at 123-134). In volume holography,the media thickness is generally greater than the fringe spacing,)

[0010] U.S. Pat. No. 6,013,454 and application Ser. No. 08/698,142, thedisclosures of which are hereby incorporated by reference, also relateto a photoimageable system in an organic polymer matrix. In particular,the application discloses a recording medium formed by polymerizingmatrix material in situ from a fluid mixture of organic oligomer matrixprecursor and a photoimageable system. A similar type of system, butwhich does not incorporate oligomers, is discussed in D. J. Lougnot etal., Pure and Appl. Optics, 2, 383 (1993). Because little or no solventis typically required for deposition of these matrix materials, greaterthicknesses are possible, e.g., 200 μm and above. However, while usefulresults are obtained by such processes, the possibility exists forreaction between the precursors to the matrix polymer and thephotoactive monomer. Such reaction would reduce the refractive indexcontrast between the matrix and the polymerized photoactive monomer,thereby affecting to an extent the strength of the stored hologram.[0011] Thus, while progress has been made in fabricating photorecordingmedia suitable for use in holographic storage systems, further progressis desirable. In particular, rapid production of high performanceholographic media which are capable of being formed in relatively thicklayers, e.g., greater than 200 μm, which substantially avoid reactionbetween the matrix material and photomonomer, and which exhibit usefulholographic properties, are desired.

SUMMARY OF THE INVENTION

[0011] This invention in high performance holographic recording articlesis based on a thermally crosslinked matrix system containingphotoimageable monomers. Fabrication of such system demanded that thearticle is of low scatter, bubble-free, optically flat, and uniform inthickness so that the articles will be of high optical quality and thatthe articles formed will have the desired dynamic range andphotosensitivity.

[0012] To meet such optical quality requirements, care must be taken todeaerate all components before their reaction is activated either byheat or catalysts. Additionally, any impurity that can produce gases asbyproducts must be eliminated to prevent bubbles formation in the curedfinal matrix system.

[0013] In addition, it is desirable that the formation of the thermallycrosslinked matrix system occurs rapidly (preferably less than 20minutes) to enable mass production of the recording media.

[0014] To prepare a high performance holographic recording article basedon two-component urethane matrix system, for example, polyols and allthe additives must be virtually free of moisture contents. Deaerationmust be carried out, once isocyanate and polyols including catalysts andall other ingredients are mixed together, to eliminate all entrapped airthat is introduced into the mixture during mixing. The deaeration takestime, and the urethane reaction must not be allowed to take place untilall air bubbles are evacuated from the isocyanate-polyols mixture. Assuch, the process limits high-volume production of the high performanceholographic recording articles.

[0015] Polyols are selected from diols and triols of polytetramethyleneglycol, polycaprolactone, polypropylene oxide. Preferred polyols arepolypropylene oxide triols with molecular weight ranging from 450 to6,000. Preferably the polyols are free of moisture contents. Hightemperature vacuum distillation treatments or additives such as moisturescavengers may be used to assure no water residue remains in the polyolsbefore use.

[0016] Additives include thermal stabilizers such as butyratedhydroxytoluene (BHT), Phenothiazine, hydroquinone, and methylether ofhydroquinone; reducers such as peroxides, phosphites, and hydroxyamines;and deformers or deaerators to eliminate entrapped air bubbles.

[0017] Tin catalysts could be used. These are dimethyltindilaurate,dibutyltindilaurate, stannous octoate, and others.

[0018] The invention constitutes an improvement over prior recordingmedia. Prior art has described thermally cross-linkedmatrix/photoimageable monomer systems but have not described methodsthat enable their rapid fabrication.

[0019] The invention's use of a matrix precursor (i.e., the one or morecompounds from which the matrix is formed) and a photoactive monomer thepolymerize by independent reactions substantially prevents bothcross-reaction between the photoactive monomer and the matrix precursorduring the cure, and inhibition of subsequent monomer polymerization.Use of a matrix precursor and photoactive monomer that form compatiblepolymers substantially avoids phase separation. And in situ formationallows fabrication of media with desirable thicknesses. The matrixprecursors are chosen to facilitate rapid production of the recordingmedia (i.e., precursors that polymerize rapidly). In addition, therecording media is fabricated through the use of an automated mixingsystem to further facilitate efficient production of the recordingmedia.

[0020] In addition to recording media, these material properties arealso useful for forming a variety of optical articles (optical articlesbeing articles that rely on the formation of refractive index patternsor modulations in the refractive index to control or modify light thatis directed at them). such articles include, but are not limited to,optical waveguides, beam steerers, and optical filters. Independentreactions indicate: (a) the reactions proceed by different types ofreaction intermediates, e.g., ionic vs. free radical, (b) neither theintermediate nor the conditions by which the matrix is polymerized willinduce substantial polymerization of the photoactive monomer functionalgroups, i.e., the group or groups on a photoactive monomer that are thereaction sites for polymerization during the pattern (e.g., hologram)writing process (substantial polymerization indicates polymerization ofmore than 20% of the monomer functional groups), and (c) neither theintermediate nor the conditions by which the matrix is polymerized willinduce a non-polymerization reaction of the monomer functional groupsthat either causes cross-reaction between monomer functional groups andthe matrix or inhibits later polymerization of the monomer functionalgroups.

[0021] Polymers are considered to be compatible if a blend of thepolymers is characterized, in 90° light scattering of a wavelength usedfor hologram formation, by a Rayleigh ratio (R_(90°)) less than 7×10⁻³cm⁻¹. The Rayleigh ratio (R_(θ)) is a conventionally known property, andis defined as the energy scattered by a unit volume in the direction θ,per steradian, when a medium is illuminated with a unit intensity ofunpolarized light, as discussed in M. Kerker, The Scattering of Lightand Other Electromagnetic Radiation, Academic Press, San Diego, 1969, at38. The Rayleigh ratio is typically obtained by comparison to the energyscatter of a reference material having a known Rayleigh ratio. Polymerswhich are considered to be miscible, e.g., according to conventionaltests such as exhibition of a single glass transition temperature, willtypically be compatible as well. But polymers that are compatible willnot necessarily be miscible. In situ indicates that the matrix is curedin the presence of the photoimageable system. A useful photorecordingmaterial, i.e., the matrix material plus the photoactive monomer,photoinitiator, and/or other additives, is attained, the materialcapable of being formed in thicknesses greater than 200 μm,advantageously greater than 500 μm, and, upon flood exposure, exhibitinglight scattering properties such that the Rayleigh ratio, R₉₀, is lessthan 7×10⁻³. (Flood exposure is exposure of the entire photorecordingmaterial by incoherent light at wavelengths suitable to inducesubstantially complete polymerization of the photoactive monomerthroughout the material.)

[0022] The optical article of the invention is formed by steps includingmixing a matrix precursor and a photoactive monomer, and curing themixture to form the matrix in situ. As discussed previously, thereaction by which the matrix precursor is polymerized during the cure isindependent from the reaction by which the photoactive monomer is laterpolymerizeduring writing of a pattern, e.g., data or waveguide form,and, in addition, the matrix polymer and the polymer resulting frompolymerization of the photoactive monomer (hereafter referred to as thephotopolymer) are compatible with each other. (The matrix is consideredto be formed when the photorecording material exhibits an elasticmodulus of at least about 10⁵ Pa. Curing indicates reacting the matrixprecursor such that the matrix provides this elastic modulus in thephotorecording material.) The optical article of the invention containsa three-dimensional crosslinked polymer matrix and one or morephotoactive monomers. At least one photoactive monomer contains one ormore moieties, excluding the monomer functional groups, that aresubstantially absent from the polymer matrix. (Substantially absentindicates that it is possible to find a moiety in the photoactivemonomer such that no more than 20% of all such moieties in thephotorecording material are present, i.e., covalently bonded, in thematrix.) The resulting independence between the host matrix and themonomer offers useful recording properties in holographic media anddesirable properties in waveguides such as enabling formation of largemodulations in the refractive index without the need for highconcentrations of the photoactive monomer. Moreover, it is possible toform the material of the invention without the disadvantageous solventdevelopment required previously.

[0023] In contrast to a holographic recording medium of the invention,media which utilize a matrix precursor and photoactive monomer thatpolymerize by non-independent reactions often experience substantialcross-reaction between the precursor and the photoactive monomer duringthe matrix cure (e.g., greater than 20% of the monomer is reacted orattached to the matrix after cure), or other reactions that inhibitpolymerization of the photoactive monomer. Cross-reaction tends toundesirably reduce the refractive index contrast between the matrix andthe photoactive monomer and is capable of affecting the subsequentpolymerization of the photoactive monomer, and inhibition of monomerpolymerization clearly affects the process of writing holograms. As forcompatibility, previous work has been concerned with the compatibilityof the photoactive monomer in a matrix polymer, not the compatibility ofthe resulting photopolymer in the matrix. Yet, where the photopolymerand matrix polymer are not compatible, phase separation typically occursduring hologram formation. It is possible for such phase separation tolead to increased light scattering, reflected in haziness or opacity,thereby degrading the quality of the medium, and the fidelity with whichstored data is capable of being recovered.

[0024] A rapid mass production methodology has been invented tofabricate these high performance holographic recording articlesincorporating compatibility, miscibility and other aforementionedadvantages of the polymer matrix and photoactive monomers. The processand composition are described below.

[0025] Additional advantages of this invention would become readilyapparent to those skilled in this art from the following detaileddescription, wherein only the preferred embodiments of this inventionare shown and described, simply by way of illustration of the best modecontemplated for carrying out this invention. As would be realized, thisinvention is capable of other and different embodiments, and its detailsare capable of modifications in various obvious respects, all withoutdeparting from this invention. Accordingly, the drawings and descriptionare to be regarded as illustrative in nature and not as restrictive.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The optical article, e.g., holographic recording medium, of theinvention is formed by steps including mixing a matrix precursor and aphotoactive monomer, and curing the mixture to form the matrix in situ.The matrix precursor and photoactive monomer are selected such that (a)the reaction by which the matrix precursor is polymerized during thecure is independent from the reaction by which the photoactive monomerwill be polymerized during writing of a pattern, e.g., data, and (b) thematrix polymer and the polymer resulting from polymerization of thephotoactive monomer (the photopolymer) are compatible with each other.As discussed previously, the matrix is considered to be formed when thephotorecording material, i.e., the matrix material plus the photoactivemonomer, photoinitiator, and/or other additives, exhibits an elasticmodulus of at least about 10⁵ Pa, generally about 10⁵ Pa to about 10⁹Pa, advantageously about 10⁶ Pa to about 10⁸ Pa.

[0027] The compatibility of the matrix polymer and photopolymer tends toprevent large-scale (>100 nm) phase separation of the components, suchlarge-scale phase separation typically leading to undesirable hazinessor opacity. Utilization of a photoactive monomer and a matrix precursorthat polymerize by independent reactions provides a cured matrixsubstantially free of cross-reaction, i.e., the photoactive monomerremains substantially inert during the matrix cure. In addition, due tothe independent reactions, there is no inhibition of subsequentpolymerization of the photoactive monomer. At least one photoactivemonomer contains one or more moieties, excluding the monomer functionalgroups, that are substantially absent from the polymer matrix, i.e., itis possible to find a moiety in the photoactive monomer such that nomore than 20% of all such moieties in the photorecording material arepresent, i.e., covalently bonded, in the matrix. The resulting opticalarticle is capable of exhibiting desirable refractive index contrast dueto the independence of the matrix from the photoactive monomer.

[0028] As discussed above, formation of a hologram, waveguide, or otheroptical article relies on a refractive index contrast (Δn) betweenexposed and unexposed regions of a medium, this contrast at least partlydue to monomer diffusion to exposed regions. High index contrast isdesired because it provides improved signal strength when reading ahologram, and provides efficient confinement of an optical wave in awaveguide. One way to provide high index contrast in the invention is touse a photoactive monomer having moieties (referred to asindex-contrasting moieties) that are substantially absent from thematrix, and that exhibit a refractive index substantially different fromthe index exhibited by the bulk of the matrix. For example, highcontrast would be obtained by using a matrix that contains primarilyaliphatic or saturated alicyclic moieties with a low concentration ofheavy atoms and conjugated double bonds (providing low index) and aphotoactive monomer made up primarily of aromatic or similar high-indexmoieties.

[0029] The matrix is a solid polymer formed in situ from a matrixprecursor by a curing step (curing indicating a step of inducingreaction of the precursor to form the polymeric matrix). It is possiblefor the precursor to be one or more monomers, one or more oligomers, ora mixture of monomer and oligomer. In addition, it is possible for thereto be greater than one type of precursor functional group, either on asingle precursor molecule or in a group of precursor molecules.(Precursor functional groups are the group or groups on a precursormolecule that are the reaction sites for polymerization during matrixcure.) To promote mixing with the photoactive monomer, the precursor isadvantageously liquid at some temperature between about −50° C. andabout 80° C. Advantageously, the matrix polymerization is capable ofbeing performed at room temperature. Also advantageously, thepolymerization is capable of being performed in a time period less than5 minutes. The glass transition temperature (T_(g)) of thephotorecording material is advantageously low enough to permitsufficient diffusion and chemical reaction of the photoactive monomerduring a holographic recording process. Generally, the T_(g) is not morethan 50° C. above the temperature at which holographic recording isperformed, which, for typical holographic recording, means a T_(g)between about 80° C. and about −130° C. (as measured by conventionalmethods).

[0030] Examples of polymerization reactions contemplated for formingmatrix polymers in the invention include cationic epoxy polymerization,cationic vinyl ether polymerization, cationic alkenyl etherpolymerization, cationic alkyl ether polymerization, cationic keteneacetal polymerization, epoxy-amine step polymerization, epoxy-mercaptanstep polymerization, unsaturated ester-amine step polymerization (viaMichael addition), unsaturated ester-mercaptan step polymerization (viaMichael addition), vinyl-silicon hydride step polymerization(hydrosilylation), isocyanate-hydroxyl step polymerization (urethaneformation), and isocyanatae-amine step polymerization (urea formation).

[0031] Several such reactions are enabled or accelerated by suitablecatalysts. For example, cationic epoxy polymerization takes placerapidly at room temperature by use of BF₃-based catalysts, othercationic polymerizations proceed in the presence of protons,epoxy-mercaptan reactions and Michael additions are accelerated by basessuch as amines, hydrosilylation proceeds rapidly in the presence oftransition metal catalysts such as platinum, and urethane and ureaformation proceed rapidly when tin catalysts are employed. It is alsopossible to use photogenerated catalysts for matrix formation, providedthat steps are taken to prevent polymerization of the photoactivemonomer during the photogeneration.

[0032] Formulation of a version of high performance holographicrecording systems comprises the following ingredients: NCO-terminatedprepolymers 20-50 Wt % Acrylate Monomers 1-15 Wt % Photoinitiators 0.2-3Wt % Polyols 40-75 Wt % Catalysts 0.1-3 Wt % Additives 0.001-0.5 Wt %

[0033] The NCO-terminated prepolymers are selected from the by-productsof diols and diisocyanates that have wt % contents of NCO in the rangeof 10 to 25. The NCO contents were determined based on the prepolymer,unreacted diisocyanate and optionally added neat polyisocyanates toachieve the high performance characteristics. Aromatic diisocynatesbased prepolymers are preferred. However, when the NCO-terminatedprepolymer is based on aliphatic diisocyanates, 5 to 100% of its wt %contents of NCO have to be derived from aromatic diisocyanates oraliphatic polyisocyanates. Preferred aromatic diisocyanates are, but nolimit to, diphenylmethane diisocyanate (MDI) and toluene diisocyanate(TDI). Preferred aliphatic polyisocyanates are: Hexamethylenediisocyanate (HDI) and its biuret, isocyanurate, uretidione, and otherderivatives.

[0034] The photoactive monomer is any monomer or monomers capable ofundergoing photoinitiated polymerization, and which, in combination witha matrix materials, meets the plymerization reaction and compatibilityrequirements of the invention. Suitable photoactive monomers includethose which polymerize by a free-radical reaction, e.g., moleculescontaining ethylenic unsaturation such as acrylates, methacrylates,acrylamides, methacrylamides, styrene, substituted styrenes, vinylnaphthalene, substituted vinyl naphthalenes, and other vinylderivatives. Free-radical copolymerizable pair systems such as vinylether mixed with maleate and thiol mixed with olefin are also suitable.It is also possible to use cationically polymerizable systems such asvinyl ethers, alkenyl ethers, alkyl ethers, ketene acetals, and epoxies.It is also possible for a single photoactive monomer molecule to containmore than one monomer functional group. As mentioned previously,relatively high index contrast is desired in the article of theinvention, whether for improved readout in a recording media orefficient light confinement in a waveguide. In addition, it isadvantageous to induce this relatively large index change with a smallnumber of monomer functional groups, because polymerization of themonomer generally induces shrinkage in a material.

[0035] Such shrinkage has a detrimental effect on the retrieval of datafrom stored holograms, and also degrades the performance of waveguidedevices such as by increased transmission losses or other performancedeviations. Lowering the number of monomer functional groups that mustbe polymerized to attain the necessary index contrast is thereforedesirable. This lowering is possible by increasing the ratio of themolecular volume of the monomers to the number of monomer functionalgroups on the monomers. This increase is attainable by incorporatinginto a monomer larger index-contrasting moieties and/or a larger numberof index-contrasting moieties. For example, if the matrix is composedprimarily of aliphatic or other low index moieties and the monomer is ahigher index species where the higher index is imparted by a benezenering, the molecular volume could be increased relative to the number ofmonomer functional groups by incorporating a naphthalene ring instead ofa benzene ring (the naphthalene having a larger volume), or byincorporating one or more additional benzene rings, without increasingthe number of monomer functional groups. In this manner, polymerizationof a given volume fraction of the monomers with the larger molecularvolume/monomer functional group ratio would require polymerization ofless monomer functional groups, thereby inducing less shrinkage. But therequisite volume fractiion of monomer would still diffuse from theunexposed region to the exposed region, providing the desired refractiveindex.

[0036] The molecular volume of the monomer, however, should not be solarge as to slow diffusion below an acceptable rate. Diffusion rates arecontrolled by factors including size of diffusing species, viscosity ofthe medium, and intermolecular interactions. Larger species tend todiffuse more slowly, but it would be possible in some situations tolower the viscosity or make adjustments to the other molecules presentin order to raise diffusion to an acceptable level. Also, in accord withthe discussion herein, it is important to ensure that larger moleculesmaintain compatibility with the matrix.

[0037] Numerous architectures are possible for monomers containingmultiple index-contrasting moieties. For example, it is possible for themoieties to be in the main chain of a linear oligomer, or to besubstituents along an oligomer chain. Alternatively, it is possible forthe index-contrasting moieties to be the subunits of a branched ordendritic low molecular weight polymer.

[0038] The preferred acrylate monomers are monofunctional. These include2,4,6-tribromophenylacrylate, pentabromoacrylate, isobornylacrylate,phenylthioethyl acrylate tetrahydrofurfurylacrylate,1-vinyl-2-pyrrolidinone, asymmetric bis thionapthyl acrylate,2-phenoxyethylacrylate, and the like.

[0039] In addition to the photoactive monomer, the optical articletypically contains a photoinitiator (the photoinitiator and photoactivemonomer being part of the overall photoimageable system). Thephotoinitiator, upon exposure to relatively low levels of the recordinglight, chemically initiates the polymerization of the monomer, avoidingthe need for direct light-induced polymerization of the monomer. Thephotoinitiator generally should offer a source of species that initiatepolymerization of the particular photoactive monomer. Typically, 0.1 to20 wt. % photoinitiator, based on the weight of the photoimageablesystem, provides desirable results.

[0040] A variety of photoinitiators known to those skilled in the artand available commercially-are suitable for use in the invention. It isadvantageous to use a photoinitiator that is sensitive to light in thevisible part of the spectrum, particularly at wavelengths available fromconventional laser sources, e.g., the blue and green lines of Ar+(458,488, 514 nm) and He—Cd lasers (442 nm), the green line of frequencydoubled YAG lasers (532 nm), and the red lines of He—Ne (633 nm) andKr+lasers (647 and 676 nm). One advantageous free radical photoinitiatorisbis(η-5-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium,available commercially from Ciba as CGI-784. Another visiblefree-radical photoinitiator (which requires a co-initiator) is5,7,diiodo-3-butoxy-6-fluorone, commercially available from SpectraGroup Limited as H-Nu 470. Free-radical photoinitiators of dye-hydrogendonor systems are also possible. Examples of suitable dyes includeeosin, rose bengal, erythrosine, and methylene blue, and suitablehydrogen donors include tertiary amines such as n-methyl diethanolamine. In the case of cationically polymerizable monomers, a cationicphotoinitiator is used, such as a sulfonium salt or an iodonium salt.These cationic photoinitiator salts absorb predominantly in the UVportion of the spectrum, and are therefore typically sensitized with adye to allow use of the visible portion of the spectrum. An example ofan alternative visible cationic photoinitiator is(η₅-2,4-cyclopentadien-1-yl) (η₆-isopropylbenzene)-iron(II)hexafluorophosphate, available commercial from Ciba as Irgacure 261. Itis also conceivable to use other additives in the photoimageable system,e.g., inert diffusing agents having relatively high or low refractiveindices.

[0041] Preferably, the photoinitiators are selected according to theirsensitivity to the light sources. For example, Irgacure 369, Irgacure819, and Irgacure 907 are suitable for commercial blue laser systems.CGI-784 is suitable for green laser systems, and CB-650 is suitable forred laser systems. Irgacure and CGI are available from Ciba, CB-650 fromSpectra Group.

[0042] Advantageously, for holographic recording, the matrix is apolymer formed by mercaptan-epoxy step polymerization, moreadvantageously a polymer formed by mercaptan-epoxy step polymerizationhaving a polyether backbone. The polyether backbone offers desirablecompatibility with several useful photoactive monomers, particularlyvinyl aromatic compounds. Specifically, photoactive monomers selectedfrom styrene, bromostyrene, divinyl benzene, and4-methylthio-1-vinylnaphthalene (MTVN) have been found to be useful withmatrix polymers formed by mercaptan-epoxy step polymerization and havinga polyether backbone. A monomer that has more than one index-contrastingmoiety and that is also useful with these polyether matrix polymers is1-(3-(naphth-1-ylthio)propylthio)-4-vinylnaphthalene.

[0043] To be independent, the polymerization reactions for the matrixprecursor and the photoactive monomer are selected such that: (a) thereactions proceed by different types of reaction intermediates, (b)neither the intermediate nor the conditions by which the matrix ispolymerized will induce substantial polymerization of the photoactivemonomer functional groups, and (c) neither the intermediate nor theconditions by which the matrix is polymerized will induce anon-polymerization reaction of the monomer functional groups that causescross-reaction (between the monomer functional groups and the matrixpolymer) or inhibits later polymerization of the monomer functionalgroups. According to item (a), if a matrix is polymerized by use of anionic intermediate, it would be suitable to polymerize the photoactivemonomer by use of a free radical reaction. In accordance with item (b),however, the ionic intermediate should not induce substantialpolymerization of the photoactive monomer, functional groups. Also inaccordance with item (b), for example, one must be aware that aphotoinitiated free radical matrix polymerization will typically inducea photoinitiated cationic polymerization of a photoactive monomerfunctional group. Thus, two otherwise independent reactions are notindependent for purposes of the invention if both are driven by a singlereaction condition. In accordance with item (c), for example,base-catalyzed matrix polymerization should not be performed when thephotoactive monomer functional group undergoes a non-polymerizationreaction in response to the base, even if polymerization of the monomerfunctional group is performed by an independent reaction. A specificexample is that a base-catalyzed epoxy-mercaptan polymerization shouldnot be used with an acrylate monomer because, although the acrylate ispolymeried by a free radical reaction, the acrylate will react with themercaptans under base catalysis, resulting in a cross-reaction.

[0044] Table 1 below illustrates some examples of matrix/photoactivemonomer combinations where the matrix polymerization reaction andphotoactive monomer polymerization are capable of being independent, andexamples where the polymerizations interfere with each other.(Photoactive monomers are horizontal, and matrix polymers are vertical.“X” indicates cross-reaction or monomer polymerization during matrixpolymerization. “O” indicates independent reactions. “I” indicates thatthe photoactive monomer polymerization is inhibited by the reagents orreaction that form the polymeric matrix, e.g., the photoactive monomerfunctional group is converted to a non-polymerizing group, or chemicalspecies are present after the matrix cure that substantially slow therate or yield of polymerization of the monomer functional groups.) TABLE1 (Meth) Styrene Vinyl acrylates Derivatives Ethers Epoxies CationicEpoxy O O X X Cationic Vinyl Ethers O O X X Epoxy (amine) X O I X Epoxy(mercaptan) X O I X Unsaturated ester (amine) X O I X Unsaturated esterX O I X (mercaptan) Hydrosilylation X X X O Urethane formation O O O X

[0045] For purposes of the invention, polymers are considered to becompatible if a blend of the polymers is characterized, in 90° lightscattering, by a Rayleigh ratio (R_(90°)) less than 7×10⁻³ cm⁻¹. TheRayleigh ratio, R_(θ), is a conventionally known property, and isdefined as the energy scattered by a unit volume in the direction θ, persteradian, when a medium is illuminated with a unit intensity ofunpolarized light, as discussed in M. Kerker, The Scattering of Lightand Other Electromagnetic Radiation, Academic Press, San Diego, 1969.The light source used for the measurement is generally a laser having awavelength in the visible part of the spectrum. Normally, the wavelengthintended for use in writing holograms is used. The scatteringmeasurements are made upon a photorecording material that has been floodexposed. The scattered light is collected at an angle of 90° from theincident light, typically by a photodetector. It is possible to place anarrowband filter, centered at the laser wavelength, in front of such aphotodetector to block fluorescent light, although such a step is notrequired. The Rayleigh ratio is typically obtained by comparison to theenergy scatter of a reference material having a known Rayleigh ratio.

[0046] Polymer blends which are considered to be miscible, e.g.,according to conventional tests such as exhibition of a single glasstransition temperature, will typically be compatible as well, i.e.,miscibility is a subset of compatibility. Standard miscibilityguidelines and tables are therefrom useful in selecting a compatibleblend. However, it is possible for polymer blends that are immiscible tobe compatible according to the light scattering test above.

[0047] A polymer blend is generally considered to be miscible if theblend exhibits a single glass transition temperature, T_(g), as measuredby conventional methods. An immiscible blend will typically exhibit twoglass transition temperatures corresponding to the T_(g) values of theindividual polymers. T_(g) testing is most commonly performed bydifferential scanning calorimetry (DSC), which shows the T_(g) as a stepchange in the heat flow (typically the ordinate). The reported T_(g) istypically the temperature at which the ordinate reaches the mid-pointbetween extrapolated baselines before and after the transition. It isalso possible to use Dynamic Mechanical Analysis (DMA) to measure T_(g).DMA measures the storage modulus of a material, which drops severalorders of magnitude in the glass transition region. It is possible incertain cases for the polymers of a blend to have individual T_(g)values that are close to each other. In such cases, conventional methodsfor resolving such overlapping T_(g) should be used, such as discussedin Brinke et al., “The thermal characterization of multi-componentsystems by enthalpy relaxation,” Thermochimica Acta., 238 (1994), at 75.

[0048] Matrix polymer and photopolymer that exhibit miscibility arecapable of being selected in several ways. For example, severalpublished compilations of miscible polymers are available, such as 0.Olabisi et al, Polymer-Polymer Miscibility, Academic Press, New York,1979; L. M. Robeson, MMI, Press Symp. Ser., 2, 177, 1982; L. A. Utracki,Polymer Alloys and Blends: Thermodynamics and Rheology, HanserPublishers, Munich, 1989; and S. Krause in Polymer Handbook, J. Brandrupand E. H. Immergut, Eds., 3rd Ed., Wiley Interscience, New York, 1989,pp. VI 347-370, the disclosures of which are hereby incorporated byreference. Even if a particular polymer of interest is not found in suchreferences, the approach specified allows determination of a compatiblephotorecording material by employing a control sample.

[0049] Determination of miscible or compatible blends is further aidedby intermolecular interaction considerations that typically drivemiscibility. For example, it is well known that polystyrene andpoly(methylvinylether) are miscible because of an attractive interactionbetween the methyl ether group and the phenyl ring. It is thereforepossible to promote miscibility, or at least compatibility, of twopolymers by using a methyl ether group in one polymer and a phenyl groupin the other polymer. It has also been demonstrated that immisciblepolymers are capable of being made miscible by the incorporation ofappropriate functional groups that can provide ionic interactions. (SeeZ. L. Zhou and A. Eisenberg, J. Polym. Sci., Polym. Phys. Ed., 21 (4),595, 1983; R. Murali and A. Eisenberg, J. Polym. Sci., Part B: Polym.Phys., 26 (7), 1385, 1988; and A Natansohn et al., Makromol. Chem.,Macromol. Symp., 16, 175, 1988). For example polyisopreme andpolystyrene are immiscible. However, when polyisoprene is partiallysulfonated (5%), and 4-vinyl pyridine is copolymerized with thepolystyrene, the blend of these two functionalized polymers is miscible.It is contemplated that the ionic interaction between the sulfonatedgroups and the pyridine group (proton transfer) is the driving forcethat makes this blend miscible. Similarly, polystyrene and poly(ethylacrylate), which are normally immiscible, have been made miscible bylightly sulfonating the polystyrene. (See R. E. Taylor-Smith and R. A.Register, Macromolecules, 26, 2802, 1993.) Charge-transfer has also beenused to make miscible polymers that are otherwise immiscible. Forexample it has been demonstrated that, although poly(methyl acrylate)and poly(methyl methacrylate) are immiscible, blends in which the formeris copolymerized with (N-ethylcarbazol-3-yl)methyl acrylate (electrondonor) and the latter is copolymerized with2-[(3,5-dinitrobenzoyl)oxy]ethyl methacrylate (electron acceptor) aremiscible, provided the right amounts of donor and acceptor are used.(See M. C. Piton and A. Natansohn, Macromolecules, 28, 15, 1995.)Poly(methyl methacrylate) and polystyrene are also capable of being mademiscible using the corresponding donor-acceptor co-monomers (See M. C.Piton and A. Natansohn, Macromolecules, 28, 1605, 1995).

[0050] A variety of test methods exist for evaluating the miscibility orcompatibility of polymers, as reflected in the recent overview publishedin A. Hale and H. Bair, Ch. 4-“Polymer Blends and Block Copolymers,”Thermal Characterization of Polymeric Materials, 2nd Ed., AcademicPress, 1997. For example, in the realm of optical methods, opacitytypically indicates a two-phase material, whereas clarity generallyindicates a compatible system. Other methods for evaluating miscibilityinclude neutron scattering, infrared spectroscopy (IR), nuclear magneticresonance (NMR), x-ray scattering and diffraction, fluorescence,Brillouin scattering, melt titration, calorimetry, andchemilluminescence. See, for example, L. Robeson, supra; S. Krause,Chemtracts-Macromol. Chem., 2, 367, 1991 a; D. Vessely in Polymer Blendsand Alloys, M. J. Folkes and P. S. Hope, Eds., Blackie Academic andProfessional, Glasgow, pp. 103-125; M. M. Coleman et al. SpecificInteractions and the Miscibility of Polymer Blends, TechnomicPublishing, Lancaster, Pa., 1991; , A. Garton, Infrared Spectroscopy ofPolymer Blends, Composites and Surfaces, Hanser, New York, 1992; L. W.Kelts et al., Macromolecules, 26, 2941, 1993; and J. L. White and P. A.Mirau, Macromolecules, 26, 3049, 1993; J. L. White and P. A. Mirau,Macromolecules, 27, 1648, 1994; and C. A. Cruz et al., Macromolecules,12, 726, 1979; and C. J. Landry et al., Macromolecules, 26, 35, 1993.

[0051] Compatibility has also been promoted in otherwise incompatiblepolymers by incorporating reactive groups into the polymer matrix, wheresuch groups are capable of reacting with the photoactive monomer duringthe holographic recording step. Some of the photoactive monomer willthereby be grafted onto the matrix during recording. If there are enoughof these grafts, it is possible to prevent or reduce phase separationduring recording. However, if the refractive index of the grafted moietyand of the monomer are relatively similar, too many grafts, e.g., morethan 30% of monomers grafted to the matrix, will tend to undesirablyreduce refractive index contrast.

[0052] A holographic recording medium of the invention is formed byadequately supporting the photorecording material, such that holographicwriting and reading is possible. Typically, fabrication of the mediuminvolves depositing the matrix precursor/photoimageable system mixturebetween two plates using, for example, a gasket to contain the mixture.The plates are typically glass, but it is also possible to use othermaterials transparent to the radiation used to write data, e.g., aplastic such as polycaronate or poly(methyl methacrylate). It ispossible to use spacers between the plates to maintain a desiredthickness for the recording medium. During the matrix cure, it ispossible for shrinkage in the material to create stress in the plates,such stress altering the parallelism and/or spacing of the plates andthereby detrimentally affecting the medium's optical properties. Toreduce such effects, it is useful to place the plates in an apparatuscontaining mounts, e.g., vacuum chucks, capable of being adjusted inresponse to changes in parallelism and/or spacing. In such an apparatus,it is possible to monitor the parallelism in real-time by use of aconventional interferometric method, and make any necessary adjustmentsduring the cure. Such a method is discussed, for example, in U.S. patentapplication Ser. No. 08/867,563, the disclosure of which is herebyincorporated by reference. The photorecording material of the inventionis also capable of being supported in other ways. For instance, it isconceivable to dispose the matrix precursor/photoimageable systemmixture into the pores of a substrate, e.g., a nanoporous glass materialsuch as Vycor, prior to matrix cure. More conventional polymerprocessing is also invisioned, e.g., closed mold formation or sheetextrusion. A stratified medium is also contemplated, i.e., a mediumcontaining multiple substrates, e.g., glass, with layers ofphotorecording material disposed between the substrates.

[0053] The medium of the invention is then capable of being used in aholographic system such as discussed previously. The amount ofinformation capable of being stored in a holographic medium isproportional to the product of: the refractive index contrast, Δn, ofthe photorecording material, and the thickness, d, of the photorecordingmaterial. (The refractive index contract, Δn, is conventionally known,and is defined as the amplitude of the sinusoidal variations in therefractive index of a material in which a plane-wave, volume hologramhas been written. The refractive index varies as: n(x)=n₀+Δn cos(K_(x)),where n(x) is the spatially varying refractive index, x is the positionvector, K is the grating wavevector, and n₀ is the baseline refractiveindex of the medium. See, e.g., P. Hariharan, Optical Holograhy:Principles, Techniques, and Applications, Cambridge University Press,Cambridge, 1991, at 44.) The Δn of a material typically calculated fromthe diffraction efficiency or efficiencies of a single volume hologramor a multiplexed set of volume holograms recorded in a medium. The Δn isassociated with a medium before writing, but is observed by measurementperformed after recording. Advantageously, the photorecording materialof the invention exhibits a Δ of 3×10⁻³ or higher.

[0054] Examples of other optical articles include beam filters, beamsteerers or deflactors, and optical couplers. (See, e.g., L. Solymar andD. Cooke, Volume Holography and Volume Gratings, Academic Press, 315-327(1981), the disclosure of which is hereby incorporated by reference.) Abeam filter separates part of an incident laser beam that is travelingalong a particular angle from the rest of the beam. Specifically, theBragg selectivity of a thick transmission hologram is able toselectively diffract light along a particular angle of incidence, whilelight along other angle travels undeflected through the hologram. (See,e.g., J. E. Ludman et al., “Very thick holographic nonspatial filteringof laser beams,” Optical Engineering, Vol. 36, No. 6, 1700 (1997), thedisclosure of which is hereby incorporated by reference.) A beam steereris a hologram that deflects light incident at the Bragg angle. Anoptical coupler is typically a combination of beam deflectors that steerlight from a source to a target. These articles, typically referred toas holographic optical elements, are fabricated by imaging a particularoptical interference pattern within a recording medium, as discussedpreviously with respect to data storage. Medium for these holographicoptical elements are capable of being formed by the techniques discussedherein for recording media or waveguides.

[0055] As mentioned previously, the material principles discussed hereinare applicable not only to hologram formation, but also to formation ofoptical transmission devices such as waveguides. Polymeric opticalwaveguides are discussed for example in B. L. Booth, “OpticalInterconnection Polymers,” in Polymers for Lightwave and IntegratedOptics, Technology and Applications, L. A. Hornak, ed., Marcel Dekker,Inc. (1992); U.S. Pat. No. 5,292,620; and U.S. Pat. No. 5,219,710, thedisclosures of which are hereby incorporated by reference. Essentially,the recording material of the invention is irradiated in a desiredwaveguide pattern to provide refractive index contrast between thewaveguide pattern and the surrounding (cladding) material. It ispossible for exposure to be performed, for example, by a focused laserlight or by use of a mask with a non-focused light source. Generally, asingle layer is exposed in this manner to provide the waveguide pattern,and additional layers are added to complete the cladding, therebycompleting the waveguide. The process is discussed for example at pages235-36 of Booth, supra, and Cols. 5 and 6 of U.S. Pat. No. 5,292,620. Abenefit of the invention is that by using conventional moldingtechniques, it is possible to mold the matrix/photoimageable systemmixture into a variety of shapes prior to matrix cure. For example, thematrix/photoimageable system mixture is able to be molded into ridgewaveguides, wherein refractive index patterns are then written into themolded structures. It is thereby possible to easily form structures suchas Bragg gratings. This feature of the invention increases the breadthof applications in which such polymeric waveguides would be useful.

[0056] The invention will be further clarified by the followingexamples, which are intended to be exemplary.

EXAMPLES AND COMPARATIVE EXAMPLES

[0057] To fabricate the high performance recording article, theNCO-terminated prepolymer and polyol must first be reacted to form amatrix in which the acrylate monomer, which remains unreacted, willreside.

[0058] As the reaction of the NCO-terminated prepolymer and polyol aretwo-component system, the NCO-terminated prepolymer, acrylate monomer,photoinitiator, and thermal stabilizers are predissolved to form ahomogeneous solution before charging into one of the holding tanks of aPosiratio two-component metering, mixing and dispensing machine,available from Liquid Control Corp. The polyol, tin catalyst, and otheradditives are premixed and charged into another holding tank. Each tankis then degassed, adjusting dispensing of materials from the tanks tothe desired amount according to the procedures outlined by LiquidControl.

[0059] Precise and accurate mixing of the two components, free ofentrapped air bubbles, is carried out by metering the liquid from bothtanks simultaneously into a helical element static mixer.

[0060] To form a holographic recording article, the desired amount ofthe well-mixed solution is dispensed onto the inner surface of thebottom substrate held by one of the parallel plate. The upper substrate,which is held by the other parallel plate, is then brought down to comein contact with the solution and held at a predetermined distance fromthe bottom plate, according to the procedures described in U.S. Pat. No.5,932,045, issued Aug. 3, 1999, the disclosure of which is herebyincorporated by reference. The entire set-up is held till the solutionbecomes solidified to assure an optically flat article is produced.

[0061] High performance holographic recording articles are characterizedby low shrinkage, dynamic range, and sensitivity. Low shrinkage willassure non-degradation of the recorded holograms and total fidelity ofthe holographic data to be recovered. Low shrinkage in the range of lessthan 0.2% is required. The dynamic range of a holographic recordingmedium is typically characterized by the parameter, M/#, a measure ofhow many holograms of a give average diffraction efficiency can bestored in a common volume. The M/# is determined by both the refractiveindex contrast and thickness of a medium. Typical values of M/# are 1.5or better. The photosensitivity is characterized by the total exposuretime required to consume the dynamic range of the media. The sensitivitycan be in the range of 25 to 120 seconds.

[0062] Details of the measurements of the recording-induced shrinkage,M/#/200 μm, and sensitivity are described in detail in Applied PhysicsLetters, Volume 73, Number 10, p. 1337-1339, 7 September 1998, which isincorporated herein by reference. Angle-multiplexing a series ofplane-wave holograms into the recording medium produce thesemeasurements. A frequency-doubled diode-pumped Nd:YAG laser used forrecording and recovery of the multiplexed holograms was spatiallyfiltered and collimated by a lens to yield a plane-wave source of light.The light was then split into two beams by polarizing beam splitters andhalf-wave plates and intersected at the sample at an external angle of44°. The power of each beam was 2 mW and the spot diameter was 4 mm.Each hologram is written with a predetermined exposure time. Afterrecording, the material was allowed to sit in the dark for 20 minutesand then flood cured with a Xenon lamp filtered to transmit wavelengthslonger than 530 nm.

Comparative Example 1

[0063] A solution was prepared containing 89.25 wt. % phenoxyethylacrylate (photoactive monomer), 10.11 wt. % ethoxylated bisphenol-Adiacrylate (photoactive monomer), 0.5 wt. % Ciba CGI-784 (identifiedpreviously) (photoinitiator), and 0.14 wt. % dibutyltin dilaurate(catalyst for matrix formation). 0.0904 g of the solution was added to avial containing 0.2784 g diisocyanate-terminated polypropylene glycol(MW=2471) (matrix precursor) and 0.05 g α,ω-dihydroxypolypropyleneglycol (MW=425) (matrix precursor). The mixture was thoroughly mixed andallowed to polymerize overnight at room temperature, while protectedfrom light. The polymerization was a step polymerization of theisocyanate groups with the hydroxyl groups to form a polyurethane withdissolved acrylate monomers. The mixture appeared clear and transparentto the naked eye. Upon exposure to an intense tungsten light, whichinitiated polymerization of the acrylate monomers, the material turnedmilky while, indicating that the polyurethane matrix and acrylatepolymers were not compatible.

[0064] Consultation of the polymer miscibility table published byKrause, referenced above, shows that polyurethanes are miscible, andthus compatible, with Saran®, a chlorinated polymer.

Comparative Example 2

[0065] The following comparative example was carried out using aPosiratio two-component metering, mixing and dispensing machine,available from Liquid Control Corp. Products in each component werepre-dissolved and mixed into a homogeneous solution, and the twosolutions were then transferred into the corresponding A and B holdingtanks of the machine. Each tank was then degassed. Dispensing ofmaterials from the tanks was adjusted to the desired amount according tothe procedures outlined by Liquid Control.

[0066] Precise and accurate mixing of the two components, free ofentrapped air bubbles, was carried out by metering the liquid from bothtanks simultaneously into a helical element static mixer. The resultantmixture, the combined weight metered out of each component, wasdispensed onto a small tin dish and observed for:

[0067] (a) Exotherm start

[0068] (b) Exotherm peak

[0069] (c) Soft gel

[0070] (d) Fabrication completed

[0071] (e) Shrinkage

[0072] (f) Dynamic range, M/#/200 μm

[0073] (g) Sensitivity, seconds to write 80% of the sample

[0074] The above mentioned properties were measured as follows:

[0075] The exotherm start, i.e., when the rise in the temperature of thematerial dispensed on the dish begins, which indicates the start of thereaction, was measured by a thermocouple or thermometer inserted withinthe material dispensed on the dish. The

[0076] The exotherm peak was recorded by monitoring the time when thetemperature of the thermocouple or thermometer peaks.

[0077] The soft-gel state was monitored by finger pressing the materialdispensed on the dish. Soft-gel state was the state when the surface ofthe material dispensed on the dish was not sticky and the material wouldnot flow when the dish was tilted vertically, but the gel could still bedeformed by finger pressing.

[0078] Solidification was also determined by finger pressing thematerial dispensed on the dish. Solidification occurred when fingerpressing could not deform the material dispensed on the dish.

[0079] The shrinkage (occurring primarily in the thickness of themedium) is determined by measuring the Bragg detuning (the shift in thereadout angle) of the angle multiplexed holograms. The quantitativerelationship between the physical shrinkage of the material and theBragg detuning is described in detail in the above reference, i.e.,Applied Physics Letters, Volume 73, Number 10, p. 1337-1339, Sep. 7,1998.

[0080] The M/# is defined to be the dynamic range of the recordingmaterial. The M/# is measured by multiplexing a series of holograms withexposure times set to consume all of the photoactive material in themedia. The M/# is defined to be the sum of the square roots of thediffraction efficiencies of all of the multiplexed holograms. Becausethe M/# depends on the thickness of the media, the quantities listed inthe examples are scaled to 200 μm thicknesses.

[0081] The sensitivity is measured by the cumulative exposure timerequired to reach 80% of the total M/# of the recording medium. Thehigher the sensitivity of the material, the shorter the cumulativeexposure time required to reach 80% of the total M/#.

[0082] The formulation of the composition for Comparative Example 2 andthe properties of the material dispensed on the dish are shown in Table2. Precise and accurate mixing of the two components, free of entrappedair bubbles, was carried out by metering the liquid from both tankssimultaneously into a helical element static mixer. The resultantmixture, the combined weight metered out of each component, wasdispensed in between two parallel 3″×3″ pieces of clear glass plates.The following times were observed for comparison:

[0083] The time it took for the exotherm to start that was defined asthe time it took for the temperature to rise from 24° C. to 30° C.

[0084] The time it took for the exotherm to peak.

[0085] The time at soft gel. At the soft-gel state, the two pieces ofglass plates could still be slid away from each other.

[0086] The time for the completion of fabrication when the mixture wassolid and the two pieces of glass could not be slid away from eachother. TABLE 2 Component 1, Tank A Baytech WE-180 415.7 gmTribromophenylacrylate 38.0 gm CGI-784 8.44 gm BHT 210 mg Component 2,Tank B Polypropylene Oxide Triol 577 gm t-Butylperoxide 310 μlDibutyltindilaurate 10.4 gm Fabricate the articles Amount metered out ofcomponent 1, Tank A 10.0 gm Amount metered out of component 2, Tank B13.0 gm Exotherm start, (temperature rises from 24° C. to 30° C.) 5 minTime for exotherm to peak 15 min. Exotherm peak temperature 39.8° C.Soft gel 35 min. Fabrication completed 3 hours Shrinkage 0.1% Dynamicrange, M/#/200 μm 2.4 Sensitivity, seconds to write 80% of the sample 25

Example 1

[0087] Samples of Example 1 were prepared and evaluated in accordancewith the procedures of Comparative Example 2 except using the followingcomponents resulting in the properties shown in Table 3 below. TABLE 3Component 1, Tank A Baytech WE-180 200 gm Mondur ML 200 gmTribromophenylacrylate 44.93 gm CGI-784 9.84 gm BHT 254 mg Component 2,Tank B Polypropylene Oxide Triol 807 gm t-Butylperoxide 310 μlDibutyltindilaurate 12.5 gm Fabricate the articles Amount metered out ofcomponent 1, Tank A 10.0 gm Amount metered out of component 2, Tank B18.0 gm Exotherm start, (temperature rises from 24° C. to 30° C.) 1 minTime for exotherm to peak 2 min Exotherm peak temperature 71° C. Softgel 3 min. Fabrication completed 17 min Shrinkage 0.12% Dynamic range,M/#/200 μm 1.8 Sensitivity, seconds to write 80% of the sample 67

Example 2

[0088] Samples of Example 2 were prepared and evaluated in accordancewith the procedures of Comparative Example 2 except using the followingcomponents resulting in the properties shown in Table 4 below. TABLE 4Component 1, Tank A Baytech WE-180 200 gm Mondur ML 200 gm Mondur TD44.4 gm Pentabromoacrylate 76.48 gm CGI-784 11.93 gm BHT 223 mgComponent 2, Tank B Polypropylene Oxide Triol 996.5 gm t-Butylperoxide474 μl Dibutyltindilaurate 11.2 gm Fabricate the articles Amount meteredout of component 1, Tank A 10.0 gm Amount metered out of component 2,Tank B 18.4 gm Exotherm start, (temperature rises from 24° C. to 30° C.)1 min Time for exotherm to peak 2 min Exotherm peak temperature 63° C.Soft gel 3 min. Fabrication completed 15 min Shrinkage 0.1% Dynamicrange, M/#/200 μm 2.1 Sensitivity, seconds to write 80% of the sample 85

Example 3

[0089] Samples of Example 3 were prepared and evaluated in accordancewith the procedures of Comparative Example 2 except using the followingcomponents resulting in the properties shown in Table 5 below. TABLE 5Component 1, Tank A Baytech MP-160 400.0 gm Mondur TD 60.0 gmTribromophenylacrylate 64.22 gm CGI-784 10.0 gm BHT 210 mg Component 2,Tank B Polypropylene Oxide Triol 750.0 gm t-Butylperoxide 398 μlDibutyltindilaurate 10.2 gm Fabricate the articles Amount metered out ofcomponent 1, Tank A 10.0 gm Amount metered out of component 2, Tank B18.7 gm Exotherm start, (temperature rises from 24° C. to 30° C.) 0.5min Time for exotherm to peak 1 min Exotherm peak temperature 56° C.Soft gel 2 min. Fabrication completed 15 min Shrinkage 0.1% Dynamicrange, M/#/200 μm 1.8 Sensitivity, seconds to write 80% of the sample 94

Example 4

[0090] Samples of Example 2 were prepared and evaluated in accordancewith the procedures of Comparative Example 2 except using the followingcomponents resulting in the properties shown in Table 6 below. TABLE 6Component 1, Tank A Baytech WE-180 180 gm Desmondur N3200 120 gmTribromoacrylate 33.9 gm BHT 188 mg Component 2, Tank B PolypropyleneOxide Triol 300 gm PMEG 1000 306 gm CGI 784 7.425 gm t-Butylperoxide 2.9ml Dibutyltindilaurate 9.47 gm Fabricate the articles Amount metered outof component 1, Tank A 1.13 gm Amount metered out of component 2, Tank B2.0 gm Exotherm start, (temperature rises from 24° C. to 30° C.) 1 minTime for exotherm to peak 5 min Exotherm peak temperature 47° C. Softgel 8 min. Fabrication completed 17 min Shrinkage 0.09% Dynamic range,M/#/200 μm 2.15 Sensitivity, seconds to write 80% of the sample 26

[0091] The above description is presented to enable a person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

[0092] This application discloses several numerical range limitations.Persons skilled in the art would recognize that the numerical rangesdisclosed inherently support any range within the disclosed numericalranges even though a precise range limitation is not stated verbatim inthe specification because this invention can be practiced throughout thedisclosed numerical ranges. A holding to the contrary would “let formtriumph over substance” and allow the written description requirement toeviscerate claims that might be narrowed during prosecution simplybecause the applicants broadly disclose in this application but thenmight narrow their claims during prosecution. Finally, the entiredisclosure of the patents and publications referred in this applicationare hereby incorporated herein by reference.

1. An optical article comprising a photoactive material and a polymermatrix formed by a polymerizing reaction of a material comprisingcomponent 1 and component 2, said component 1 comprises a NCO-terminatedpre-polymer and said component 2 comprises a polyol; wherein saidmaterial has an exotherm peak occurring within 12 minutes after mixingsaid component 1 and said component
 2. 2. The optical article of claim1, wherein the polymerization reaction is selected from a groupconsisting of a urethane formation reaction, an urea formation reactionand combinations thereof.
 3. The optical article of claim 1, whereinsaid component 1 comprises a substance selected from the groupconsisting of aromatic isocyanate, aliphatic isocyanate and combinationsthereof.
 4. The optical article of claim 1, wherein said component 1comprises a substance selected from the group consisting of aromaticdiisocyanate, hexamethylene diisocyanate, a derivative of hexamethylenediisocyanate and combinations thereof.
 5. The optical article of claim1, wherein said polyol comprises a polyol of polypropylene oxide.
 6. Theoptical article of claim 5, wherein said component 2 further comprises apolytetramethylene ether diol.
 7. The optical article of claim 1,wherein the photoactive material is a photoactive monomer.
 8. Theoptical article of claim 7, wherein the photoactive monomer is anacrylate monomer.
 9. The optical article of claim 1, wherein the opticalarticle is a holographic recording medium having a thickness greaterthan 200 μm and Δn of 3×10⁻³ or higher.
 10. The optical article of claim1, wherein the optical article is an optical waveguide.
 11. The opticalarticle of claim 1, wherein the optical article has a writing inducedshrinkage of less than 0.25 percent.
 12. The optical article of claim 1,wherein the optical article is a holographic recording medium.
 13. Theoptical article of claim 1, wherein said material has an exotherm peakoccurring within 5 minutes after mixing said component 1 and saidcomponent 2 and said material has a soft gel within 15 minutes aftermixing said component 1 and said component
 2. 14. The optical article ofclaim 1, wherein said material has an exotherm peak occurring within 3minutes after mixing said component 1 and said component 2 and saidmaterial has a soft gel within 5 minutes after mixing said component 1and said component
 2. 15. An optical article comprising a photoactivematerial and a polymer matrix formed by a polymerizing reaction of amaterial comprising component 1 and component 2, said component 1comprises biscyclohexylmethane diisocyanate, a NCO-terminated prepolymerformed by a reaction of biscyclohexylmethane diisocyanate andpolytetramethylene glycol, butyrated hydroxytoluene and a derivative ofhexamethylene diisocyanate, and said component 2 comprises a polyol ofpolypropylene oxide and a polyol of polytetramethylene ether; whereinsaid material has an exotherm peak occurring within 12 minutes aftermixing said component 1 and said component
 2. 16. An optical articlecomprising a photoactive material and a polymer matrix formed by apolymerizing reaction of a material comprising component 1 and component2, said component 1 comprises a NCO-terminated pre-polymer selected fromthe group consisting of diphenylmethane diisocyanate, toluenediisocyanate, hexamethylene diisocyanate and a derivative ofhexamethylene diisocyanate, and said component 2 comprises a polyol ofpolypropylene oxide; wherein said material has an exotherm peakoccurring within 12 minutes after mixing said component 1 and saidcomponent
 2. 17. A method of manufacturing an optical article,comprising: mixing component 1 and component 2 to form a materialcomprising a photoactive material and reacting ingredients of saidmaterial, wherein said component 1 comprises a NCO-terminatedpre-polymer and said component 2 comprises a polyol, and said materialhas an exotherm peak occurring within 12 minutes after mixing saidcomponent 1 and said component
 2. 18. The method of claim 17, whereinsaid reacting comprises a polymerization reaction selected from a groupconsisting of a urethane formation reaction, an urea formation reaction,cationic epoxy polymerization, cationic vinyl ether polymerization,cationic alkenyl ether polymerization, cationic allyl etherpolymerization, cationic ketene acetal polymerization, epoxy-amine steppolymerization, epoxy-mercaptan step polymerization, unsaturatedester-amine step polymerization, unsaturated ester-mercaptan steppolymerization, a hydrosilylation reaction and combinations thereof. 19.The method of claim 17, wherein said component 1 comprises a substanceselected from the group consisting of aromatic isocyanate, aliphaticisocyanate and combinations thereof.
 20. The method of claim 17, whereinsaid component 1 comprises a substance selected from the groupconsisting of aromatic diisocyanate, hexamethylene diisocyanate, aderivative of hexamethylene diisocyanate and combinations thereof. 21.The method of claim 17, wherein said polyol comprises a polyol ofpolypropylene oxide.
 22. The method of claim 21, wherein said component2 further comprises a polytetramethylene ether diol.
 23. The method ofclaim 17, wherein the photoactive material is a photoactive monomer. 24.The method of claim 23, wherein the photoactive monomer is an acrylatemonomer.
 25. The method of claim 17, wherein said material has anexotherm peak occurring within 5 minutes after mixing said component 1and said component 2 and said material has a soft gel within 15 minutesafter mixing said component 1 and said component
 2. 26. The method ofclaim 17, wherein said material has an exotherm peak occurring within 3minutes after mixing said component 1 and said component 2 and saidmaterial has a soft gel within 5 minutes after mixing said component 1and said component
 2. 27. A method of manufacturing an optical article,comprising: mixing a photoactive material and a material comprisingcomponent 1 and component 2, said component 1 comprisesbiscyclohexylmethane diisocyanate, a NCO-terminated prepolymer formed bya reaction of biscyclohexylmethane diisocyanate and polytetramethyleneglycol, butyrated hydroxytoluene and a derivative of hexamethylenediisocyanate, and said component 2 comprises a polyol of polypropyleneoxide and a polyol of polytetramethylene ether, and reacting ingredientsof said material; wherein said material has an exotherm peak occurringwithin 12 minutes after mixing said component 1 and said component 2 andsaid material has a soft gel within 30 minutes after mixing saidcomponent 1 and said component
 2. 28. A method of manufacturing anoptical article, comprising: mixing a photoactive material and amaterial comprising component 1 and component 2, said component 1comprises a NCO-terminated pre-polymer selected from the groupconsisting of diphenylmethane diisocyanate, toluene diisocyanate,hexamethylene diisocyanate and a derivative of hexamethylenediisocyanate, and said component 2 comprises a polyol of polypropyleneoxide, and reacting ingredients of said material; wherein said materialhas an exotherm peak occurring within 12 minutes after mixing saidcomponent 1 and said component 2 and said material has a soft gel within30 minutes after mixing said component 1 and said component 2.